Composites of polysiloxane polymers and inorganic nanoparticles

ABSTRACT

Desirable composites of polysiloxane polymers and inorganic nanoparticles can be formed based on the appropriate selection of the surface properties of the particles and the chemical properties of the polymer. High loadings of particles can be achieved with good dispersion through the polymer. The composites can have good optical properties. In some embodiments, the inorganic particles are substantially free of surface modification.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No.61/143,206, filed on Jan. 8, 2009, to Du et al., entitled “Composites ofPolysiloxane Polymers and Inorganic Nanoparticles,” incorporated hereinby reference.

FIELD OF THE INVENTION

The invention relates to composites of polysiloxane polymers andinorganic nanoparticles, especially in which the nanoparticles are welldispersed within the polymer matrix. The invention further relates tomethod for forming composites of polysiloxane polymers and inorganicnanoparticles.

BACKGROUND OF THE INVENTION

Composites can be desired materials for a range of applications becausethey combine desirable properties of the individual materials and canprovide unique properties relative to the individual materials. Polymersgenerally can provide a range of advantageous processing approacheswhile providing a reasonable range of available properties through theselection of the polymer composition. Similarly, inorganic materials canintroduce various desirable mechanical and physical properties. Ingeneral, commercial applications provide growing demands on materialproperties.

SUMMARY OF THE INVENTION

In a first aspect, the invention pertains to a composite compositioncomprising a first polysiloxane polymer and inorganic particles, whereinthe inorganic particles have an average particle size of no more thanabout 250 nm and are substantially free of organic surface modificationdistinct from the polysiloxane polymer. In some embodiments, theinorganic particles comprise a metal oxide, a metal nitride or acombination thereof.

In a further aspect, the invention pertains to a method for forming acomposite, the method comprising the step of combining well dispersedinorganic particles in a dispersant liquid with a first polysiloxanepolymer in a suitable solvent for solubilizing the first polysiloxanepolymer to form a blend. Generally, the inorganic oxide particles havean average particle diameter of no more than about 250 nm andsubstantially no organic surface modification. In some embodiments theinorganic particles comprise a metal oxide, a metal nitride or acombination thereof.

In another aspect, the invention pertains to a composite compositioncomprising a first polysiloxane polymer, a second polysiloxane polymerand inorganic particles. Generally, the inorganic particles have anaverage particle size of no more than about 250 nm. In some embodiments,the first polysiloxane polymer has compatible functional groups thathave stable interactions with the inorganic particles, and the secondpolysiloxane polymer does not have compatible functional groups. In someembodiments, the inorganic particles comprise a metal oxide, a metalnitride or a combination thereof.

In additional aspects, the invention pertains to a method for theformation of a composite, the method comprising the steps of combiningwell dispersed inorganic particles with a first polysiloxane polymer ina suitable solvent for solubilizing the first polysiloxane polymer toform a blend and combining a second polysiloxane polymer with the blendof the inorganic particles and the first polysiloxane polymer.Generally, the inorganic oxide particles have an average particlediameter of no more than about 250 nm. The first polysiloxane polymercan comprise compatible functional groups that have stabilizinginteractions with the inorganic particles. In some embodiments, theinorganic particles comprise a metal oxide, a metal nitride or acombination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of scattering intensity as a function of particlediameter based on an optical measurement of secondary particle size in adispersion of titanium oxide particles that have not been surfacemodified.

FIG. 2 is a plot of percent transmittance as a function of lightwavelength for three polysiloxane films in which films 2 and 3 arecomposites with a 50 weight percent (film 2) or 60 weight percent (film3) titanium oxide particles.

FIG. 3 is a plot of scattering intensity as a function of particlediameter based on an optical measurement of secondary particle size in adispersion of surface modified titanium oxide particles in which thesurface modification comprises hydrophobic organic groups.

FIG. 4 is a plot of percent transmittance for composites withpolysiloxane and surface modified titanium oxide formed with the surfacemodified particles used to obtain the data in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Significant loadings of inorganic particles can be introduced intocomposites formed with siloxane polymers with a high level of uniformityof the particles within the composite. In general, it is difficult tocombine inorganic particles with some siloxane polymers due to thechemical incompatibility of the two materials. However, it has beenfurther found that introduction of appropriately selected functionalgroups onto the siloxane polymers can improve the compatibility of theinorganic particles with the polymer so that well blended composites canbe formed at loading up to high levels. The selected functional groupscan be added as terminal groups and/or branch groups as well as inassociation with certain monomers within a copolymer and/or a polymerblend. The inorganic particles may or may not be surface modified. Inparticular, it has been surprisingly found that inorganic particleswithout substantial organic surface modification can be incorporatedinto appropriate siloxane polymers at high particle loadings using themethods described herein. In some embodiments, a polymer blend cancomprise an oligomer or oligomers selected to be chemically compatiblewith the inorganic particles. The dispersion of the inorganic particleswithin the polysiloxane polymer can be accomplished at a level thatprovides for very good optical properties of the resulting composites,although the composites can be used for a range of applications alongwith optical applications.

For composites formed with polydimethylsiloxane, it has been found thatapproaches based on the surface modification of the inorganic particlesgenerally have not achieved the desired results with respect to highinorganic particle loadings with good dispersion of the inorganicparticles. It has been discovered that the appropriate engineering ofthe polymer properties with the surface properties of the inorganicparticles provides for good dispersion of the inorganic particles withinthe polysiloxane polymers, which can be maintained even at highloadings. Thus, the composites described herein comprise polysiloxanepolymers or oligomers with compatible functional groups that stabilizethe inorganic particles within the polymer matrix to form a well blendedcomposite. The functional groups for the polymers can be introducedthrough the selection of the particular polymer, certain monomers withina copolymer and/or through the use of a polymer blend. Similarly, theinorganic particles can be surface modified to introduce appropriatesurface chemistry to the inorganic particles.

Polysiloxane polymers, which are also referred to as silicone, havefound commercial applicability based on the flexibility of the polymers.Silicone rubbers and other polysiloxane resins are used to formencapsulants, coatings, and the like. In general, these polymers have ahigh degree of thermal stability, oxidative stability and chemicalresistance. Inorganic particles can introduce desired mechanical,optical or functional properties to the composite, while exploiting thedesired properties of the polysiloxanes. The polysiloxane polymers canbe crosslinked or cured to provide desired physical properties followingthe association with the inorganic particles.

It has been discovered that highly dispersed inorganic particles withina composite can result in a composite material that has the appearancewith respect to certain properties of a homogenous material with thehybrid properties of the composite. In general, a particular propertyhas a distance scale associated with that property that determines thedistance scale on which the dispersion of the particles within thepolymer matrix should appear uniform for the material to look relativelyhomogenous with respect to that property. Thus, it is desirable for theparticles to be relatively non-aggregated such that the clusters are nottoo large. Also, it is desirable for the particles to be relativelyuniformly dispersed. Then, the composite can be transparent with littlescattering. Some lack of uniformity in the particle dispersion canresult in some scattering or other corresponding appearance ofinhomogeneity. Improved optical properties as a result of more uniformdispersion of inorganic particles within a polymer composite isdescribed further in published U.S. Patent Application 2008/0150184A toChiruvolu et al., entitled “Composites of Polymers and Metal/MetalloidOxide Nanoparticles and Methods for Forming These Composites,”incorporated herein by reference.

The composites comprise inorganic particles or a combination thereofdispersed within a polysiloxane polymer or a combination thereof. Thecomposite can further comprise relatively small amounts of additives,such as antioxidants, viscosity modifiers and the like. In general,values of the inorganic particle loading within the composite can beselected based on the composite properties desired for the selectedapplication and the properties of the various components of thecomposite. In some embodiments, low inorganic particle loadings aresufficient. However, in some embodiments, the composite comprises atleast about 5 weight percent inorganic particles, in further embodimentsat least about 10 weight percent, in other embodiments from about 20weight percent to about 75 weight percent inorganic particles and inadditional embodiments from about 25 weight percent to about 70 weightpercent. A person of ordinary skill in the art will recognize thatadditional ranges of particle loadings within the explicit ranges aboveare contemplated and are within the present disclosure.

In some embodiments, the composite comprises a plurality of polysiloxanepolymers, in which the distinct polymers have different chemicalcompositions. Specifically, the polymer blend can comprise 2 distinctpolymers, 3 distinct polymers or more than 3 distinct polymers. Forexample, one polysiloxane polymer can have a functional group thatfacilitates formation of a uniform composite with the appropriateinorganic particles. The use of a polymer blend can provide for overalldesirable polymer properties while also providing for relatively uniformincorporation of the inorganic particles into the composite.

Generally, for polymer blends the total amount of polysiloxane polymercomprises at least about 15 weight percent polysiloxane polymercompatible with the inorganic particles, in other embodiments at leastabout 20 weight percent, in further embodiments from about 25 weightpercent to about 95 weight percent and in additional embodiments fromabout 30 weight percent to about 80 weight percent polysiloxane polymercompatible with the inorganic particles. In some embodiments, one of thepolysiloxane polymers of the blend can be an oligomer, i.e., a lowermolecular weight polymer, as described further below. The oligomer canhave functional groups such that the oligomer is compatible with theinorganic particles. In embodiments with polysiloxane oligomers, thecomposite can generally comprise at least about 1 weight percentoligomer, in further embodiments at least about 3 weight percentoligomer, in other embodiments from about 4 weight percent to about 80weight percent oligomer, in additional embodiment from about 5 weightpercent to about 60 weight percent oligomer and in some embodiments fromabout 7 weight percent to about 25 weight percent oligomer. A person ofordinary skill in the art will recognize that additional ranges withinthe explicit concentration ranges above are contemplated and are withinthe present disclosure.

With respect to oligomers with desirable functional groups, it is notedthat polysiloxane oligomers with epoxy groups are commerciallyavailable. For example, an epoxy-siloxane oligomer UV 9400 is availablefrom GE Silicones. The epoxy group provides a polar group to the polymerblend, and the epoxy group can provide for subsequent crosslinking.Epoxy silanes provide similar properties. It has also been found thatblends of polysiloxanes with appropriate amounts of epoxy-silane arecompatible and blend well with metal oxide particles as described hereinwithout any further surface modification.

The inorganic particles may or may not be surface modified forincorporation into a polymer composite. A surface modifying agent may ormay not chemically bond with the inorganic particle. If the surfacemodification agent does not chemically bond with the particles, thesurface modification agent may associate with the surface due tohydrogen bonding, electrostatic attraction, non-specific interactionsand/or entropic effects. As used herein, chemical bonding refers tobonds with some significant covalent character, which can include, forexample, bonds found in organic compositions, metal-ligand bonds and thelike. Certain functional groups have the ability to form chemical bondswith inorganic particles. These functional groups can form the basis forbonding surface modification agents and/or polymers to the inorganicparticle surfaces.

Un-modified inorganic particles, i.e., without any substantial surfacemodification with an organic composition, can have particular surfaceproperties, which may depend on the method of synthesis and thetreatment of the particles following synthesis. However, metal oxideparticles, metal nitride particles and other inorganic particles canhave a polar surface, such as if the surface has terminal hydroxide oramine groups. Thus, polar functional groups would be expected to havestabilizing interactions with these inorganic particles if they have apolar surface. If the particles are surface modified, the surfacemodification agent introduces different functionality to the inorganicparticle surface. This functionality can be polar, non-polar orhydrophobic, aromatic or the like. The particles are free of substantialmodification if the particles are unmodified or have a very low degreeof surface modification such that the properties of the particles indispersions are unchanged based on any reasonable measurement from thecompletely unmodified form of the particles.

In general, desirable inorganic particles for the composites aresubmicron, i.e., the particle collections generally have an averageprimary particle diameter of no more than about 1 micron, which is equalto 1000 nanometers (nm), in some embodiments no more than about 250 nm,in further embodiments no more than about 100 nm, in other embodimentsfrom about 2 nm to about 50 nm and in additional embodiments from about2 nm to about 25 nm. A person of ordinary skill in the art willrecognize that additional ranges of average particle diameter within theexplicit range above are contemplated and are within the presentdisclosure.

In some embodiments, the particles are very uniform in particle sizesuch that the particle collections have corresponding uniformity ofproperties. In particular, laser pyrolysis as well as some othersynthesis methods can produce nanoparticles having a very narrow rangeof particle diameters. As determined from examination of transmissionelectron micrographs, the particles generally have a distribution insizes such that at least about 95 percent, and in some embodiments 99percent, of the particles have a diameter greater than about 35 percentof the average diameter and less than about 280 percent of the averagediameter. In additional embodiments, the particles generally have adistribution in sizes such that at least about 95 percent, and in someembodiments 99 percent, of the particles have a diameter greater thanabout 40 percent of the average diameter and less than about 250 percentof the average diameter. In further embodiments, the particles have adistribution of diameters such that at least about 95 percent, and insome embodiments 99 percent, of the particles have a diameter greaterthan about 60 percent of the average diameter and less than about 200percent of the average diameter. A person of ordinary skill in the artwill recognize that other ranges of uniformity within these specificranges are contemplated and are within the present disclosure.

Furthermore, in some embodiments essentially no particles have anaverage diameter greater than about 5 times the average diameter, inother embodiments about 5 times the average diameter, in furtherembodiments 4 times the average diameter, and in additional embodiments3 times the average diameter. In other words, the particle sizedistribution effectively does not have a tail indicative of a smallnumber of particles with significantly larger sizes. This is a result ofthe small reaction region to form the inorganic particles andcorresponding rapid quench of the inorganic particles. An effective cutoff in the tail of the size distribution such that essentially noparticles exceed a cutoff value indicates that there are less than about1 particle in 10⁶ has a diameter greater than a specified cut off valueabove the average diameter. High particle uniformity can be exploited toform more uniform composite compositions.

In some embodiments, the composites comprise a mixture of inorganicparticles that can be selected to contribute different properties to theultimate composite. The inorganic particles generally can comprise metalelements in compounds. Specifically, the inorganic particles cancomprise, for example, metal oxides, metal nitrides or combinationsthereof. Metal oxide compounds are of particular interest for thecomposites described herein. As used herein, the term metal includessilicon, even though silicon may be considered a metalloid. Thus,silicon oxide and silicon nitride are respectively considered a metaloxide and a metal nitride.

Suitable nanoparticles can be formed, for example, by vapor-based flowprocesses, such as flame pyrolysis or the like, or solution-basedapproaches, such as sol gel approaches. Vapor-based particle productiontechniques in a flow can be desirable due to their flexibility withrespect to product particle composition, ability to form highlycrystalline particle either directly or with mild additional processingand a wide range of ability to introduce dopants. However, the resultingdry powders of particles can be more difficult to process. Inparticular, these particles are more difficult to disperse in a liquid.In contrast, solution-based synthesis approaches can form particles thatare inherently more dispersible, but the particles can have lessdesirable properties due to decreased levels of crystallinity, limitedcompound selection and difficulties in selecting among different crystalstructures.

Laser pyrolysis has been successfully used for the synthesis of a widerange of complex inorganic particles, including, for example,compositions with multiple metal elements as well as doped materials.For example, phosphor particles with desirable properties have beenproduced. See, for example, U.S. Pat. No. 6,692,660 to Kumar, entitledHigh Luminescent Phosphor Particles and Related Particle Compositions,”incorporated herein by reference. Also, high quality rutile titaniumoxide particles are suitable for forming composites with a high index ofrefraction. The formation of rutile titanium dioxide with a high degreeof particle uniformity is described further in U.S. Pat. No. 6,599,631to Kambe et al., entitled Polymer Inorganic Particle Composites,”incorporated herein by reference. The synthesis of a wide range ofuniform inorganic nanoparticles using laser pyrolysis is describedfurther in Published U.S. Patent Application 2003/0203205 to Bi et al.,entitled “Nanoparticle Production and Corresponding Structures,”incorporated herein by reference. Flame spray pyrolysis has beendemonstrated as an effective approach for the synthesis of metal oxidenano-particles having good uniformity. Flame spray pyrolysis has beendescribed for the synthesis of uniform metal oxides in U.S. patentapplication Ser. No. 12/288,890 to Jaiswal et al., entitled “Flame SprayPyrolysis With Versatile Precursors for Metal Oxide NanoparticleSynthesis and Applications of Submicron Inorganic Oxide Compositions forTransparent Electrodes,” incorporated herein by reference.

Metal nitrides can be formed using laser pyrolysis through theintroduction of a nitrogen source along with the exclusion ofsignificant oxygen sources. For example, suitable nitrogen sourcesinclude, for example, N₂ or NH₃. The synthesis of Si₃N₄ using laserpyrolysis with silane and ammonia precursors is described in publishedPCT patent application WO 01/32799 to Reitz et al., entitled “ParticleDispersions,” incorporated herein by reference. Other metal nitrides canbe similarly synthesized.

For crystalline particles, the surface of the particle represents anabrupt termination of the crystal structure. The surface chemistry ofthe particles and the structure of the crystal at its surface arecorrelated with each other. Conceptually, simply ending the crystalstructure abruptly results in free radicals or similar unstablestructures due to dangling bonds. While some radicals can be stable,generally the particle surface rearranges to form chemically stablespecies which can have surface strain and alteration of the crystalstructure near the surface.

For example, with metal oxide particles, the structure along theparticle surface can have bridging oxygen atoms (—O—) or double bondedoxygen atoms (═O) to terminate the crystal structure with appropriatechemical stability. However, these structures tend to introduce astrained but relatively inert surface. Alternatively or additionally,the presence of single valent atoms, such as H or a halogen, canterminate the crystal structure with stable groups, such as M—Cl orM—O—H, where M is a metal atom of the crystal. Truncation of the crystallattice at mono-valent atoms can reduce surface strain, result in lesscrystal restructuring near the particle surface and provide surfacegroups for later surface modification. As a result of the surfaceproperties of these particles, they are generally better dispersed inpolar liquids. Contacting the particles with water during theirsynthesis or subsequent to their synthesis can facilitate the formationof surface OH groups on metal oxide particle surfaces.

Because of their small size, nanoparticles tend to form looseagglomerates due to van der Waals and other electromagnetic forcesbetween nearby particles. These loose agglomerates can be dispersed in adispersant to a significant degree. The primary particle size, ofcourse, is the lower limit of the secondary particle size for aparticular collection of particles, so that the average secondaryparticle size can be approximately the average primary particle size ifthe primary particles are substantially unfused and if the particles areeffectively completely dispersed in the liquid, although measurements ofsecondary particle size involves an indirect measurement that can beinfluenced by solvation effects.

Secondary particle size refers to the particle size in a dispersion. Thesecondary or agglomerated particle size may depend on the subsequentprocessing of the particles following their initial formation and thecomposition and structure of the particles. Secondary particles sizeswithin a liquid dispersion can be measured by established approaches,such as dynamic light scattering. Suitable particle size analyzersinclude, for example, a Microtrac UPA instrument from Honeywell based ondynamic light scattering, a Horiba Particle Size Analyzer from Horiba,Japan and ZetaSizer Series of instruments from Malvern based on PhotonCorrelation Spectroscopy. The principles of dynamic light scattering andother optical approaches for particle size measurements in liquids arewell established.

The ability of a liquid to disperse particles within the liquid dependson the surface properties of the particles, the nature of the liquiddispersant, the concentration of particles and the process used todisperse the particles as well as the physical particle size. Higherconcentrations of particles tend to favor agglomeration due to basicthermodynamic principles. However, proper selection ofsolvent/dispersant properties based on the surface properties of theparticles can facilitate better dispersion. Similarly, the applicationof shear, sound waves and/or other mixing/disruptive forces canfacilitate dispersion of the particles. The high quality inorganicparticles disclosed herein under appropriate conditions can be dispersedto have secondary particle sizes approximately a few times larger thanthe primary particle sizes. These excellent dispersion properties can beaccomplished at moderately high loadings for dry synthesized metal oxidepowders, such as up to 5 weight percent inorganic powders or even higherconcentrations.

It has been found that metal oxide particles produced by flow-basedsynthesis approaches can be well dispersed at moderate concentrations inunmodified form in alcohols, such as methanol or propylene glycol. Theas-synthesized particles can be stably dispersed in water at moderateconcentrations, although the average secondary particle size generallyis larger than in dispersions in certain alcohols. Also, it has beendiscovered that the contact of the particles with water also canincrease the presence of —OH groups on the surface of the particles. Theincreased presence of —OH groups improves the subsequent dispersion ofthe particles in alcohols and provides additional functional groups forbonding with surface modification agents, such as alkoxysilanes. Thus,the initial contact of the inorganic particles with water and thesubsequent dispersion in a suitable alcohol results in a surprisinglyimproved dispersion in the alcohol. Improved dispersions are stable witha relatively small average secondary particle size at a particularconcentration.

Results for dynamic light scattering (DLS) measurements of secondaryparticle size can be reported as an intensity average, referred to asthe Z-average or cumulants mean. Alternatively, volume average particlesizes can be used to evaluate secondary particle sizes, althoughz-average values are generally less variable in measurements. In someembodiments, the secondary particles have a z-average size no more thanabout 1000 nm, in additional embodiments no more than about 500 nm, infurther embodiments from about 2 nm to about 300 nm, in otherembodiments about 2 nm to about 100 nm, and alternatively about 2 nm toabout 50 nm. A person of ordinary skill in the art will recognize thatother ranges within these specific ranges are contemplated and arewithin the present disclosure.

In general, the metal oxide particles formed by laser pyrolysis can bewell dispersed in water or alcohols at moderate concentrations with nosurface modification. Suitable alcohols include, for example, smallaliphatic alcohols, such as methanol, ethanol, propylene glycol,butanediol and the like. These alcohols generally form betterdispersions than water at the same concentrations. Better dispersionsare more stable and/or have a smaller secondary particle size indicatingless agglomeration. In general, dispersions with well dispersedparticles can be formed at concentrations of up to 15 weight percentinorganic particles, in other embodiments from about 0.25 to about 10weight percent and in further embodiments from about 0.5 to about 9weight percent. In some embodiments, for dispersions with well dispersedparticles, the average secondary particle size can be no more than afactor of four times the average primary particle size, in furtherembodiments no more than about 3 times the average primary particle sizeand in additional embodiments no more than about 2 times the averageprimary particle size.

The surface modification of the inorganic particles can improvestability of the particle dispersions, provide for dispersion of theparticles in a wider range of liquids and provide for desired processingflexibility for the formation of composites as well as facilitating theuniform dispersion of the inorganic particles with a wider range ofpolymers. While surface modifiers can merely coat the surface, improvedstability of the coated particles is accomplished with surface modifiersthat are chemically bonded to the surface. In particular, alkoxysilanesreact with metal oxides to form silicon-O-metal bonds to form a stablesurface coating with the release of a corresponding compound from thedisplaced silane functional group. An improved surface coating isachieved with improved —OH functional group coverage on the surface ofthe metal oxide particles. The surface modification process can involvea switch of dispersants. For convenience of terminology, a surfacemodifying compound refers to a compound that adds at least 3 atoms tothe particle surface when it bonds to the particle surface, todistinguish compositions, such as water, that modify the surface of ametal oxide particle such as through the substitution of an —OH group.

In particular, trialkoxysilanes provide very stable bonding to metaloxide particle surfaces with potentially three points of bonding. Thefourth side chain of the trialkoxysilanes provides the ability toinfluence the dispersability and other surface properties of the surfacemodified inorganic particles. Specifically, the fourth side chain of thesilane can be selected to improve dispersability in a selected solventand/or to provide a reactive functional group for further processing.Alternatively or additionally, the fourth side chain can be used tostabilize the interaction of the surface modified particle with aselected polymer so that a uniform composite can be formed. Similarly,polydialkoxy siloxy silanes provide stable bonding with the ability ofeach monomer unit to form two bonds to the particle. The polymer canwrap around the particles during the bonding process. In addition toalkoxy silanes, chemical compounds with other functional groups can formbonds to metal oxide particles. Specifically, compounds withchlorosilicate (—SiCl) groups, some amine groups, carboxylic acid groupsand hydroxide groups can also bond to metal oxide particle surfaces.These compounds can also be expected to bond with the surfaces of metalnitride particles. Similarly, carbonyl groups, hydroxide groups andamine groups of surface modifying compounds can be expected to bond withthe surfaces of metal oxide and metal nitride particles.

Two processes can be used to perform the surface modification. In oneapproach, an unstable, high concentration dispersion can be formed withthe particles, and the surface modification is performed to stabilizethe high concentration dispersion. However, better particle dispersionsgenerally are obtained through first forming a dilute, stabiledispersion of the particles without surface modification and thenperforming the surface modification. As noted above, alcohols,especially propylene glycol, and water/alcohol blends are gooddispersants for the unmodified metal oxide particles. The surfacemodifying compound can be added directly into the alcohol orwater/alcohol blend if it has some solubility, or the surfacemodification compound can be dissolved into a solvent that is misciblewith or soluble in the liquid of the particle dispersion. After thesurface modification is complete, the particles can be precipitated fromthe dispersant by mixing a suitable liquid into the dispersion that issoluble or miscible with the dispersant used to perform the surfacemodification, and then re-suspended in a desired dispersant. The surfacemodified particles can be stored or shipped in a liquid suitable forfurther processing.

Polysiloxane polymers can also be referred to as silicones. Polysiloxanepolymers have a chemical structure represented by T₁-(OSiR₁R₂)_(n)—OT₂where R₁ and R₂ are independently organic side groups and T₁ and T₂ areterminal groups. In some embodiments, R₁ and R₂ are the same, while inother embodiments, R₁ and R₂ are different. Commercially availablepolymers include, for example, polydimethyl siloxane, polydiphenylsiloxane and polymethylphenyl siloxane. Some commercial siloxanepolymers have polar groups such that the polymers are soluble in polarsolvents, and one such polar polysiloxane is discussed in the Examplesbelow.

The repeat number “n” generally is a distribution relating to thedistribution of polymer chain lengths that depends on the polymerizationconditions. The average value of n, <n>, is at least 2, and in someembodiments least 12, in other embodiments at least 15 and in furtherembodiments at least 20. The value of <n> generally can be estimatedfrom a measurement of the average molecular weight of the polymer.Oligomers as used herein refers to polysiloxane polymers with thestructure R₁R₂R₃Si—(OSiR₄R₅)_(n)OSiR₆R₇R₈, where R₁-R₈ are independentlyorganic groups, wherein 0<=n<=10. The desired values of <n> andcorrespondingly the average polymer molecular weight can be selectedbased on the particular application and the desired properties of thecomposite for that application.

The polymers can be crosslinked, which involves chemical bonds betweenpolymer side chains. The degree of crosslinking affects the physicalproperties of the polymers. In general, the polymers can be crosslinkedfollowing the formation of the composite such that the ultimate productcan have a component comprising a durable composite component.

In some embodiments, polysiloxane polymers of interest have compatiblefunctional groups that have stabilizing attractions with the inorganicparticles of the composite. The nature of a compatible functional groupis described further below. The compatible functional group can be atthe terminal positions of the polymer. In other words, T₁ and/or T₂ havepolar functional groups. If the polysiloxane polymers are synthesizedthrough the hydrolysis of halides, SiR₁R₂X₂, the polymerization islimited by the availability of monomers, and the resulting terminalgroups are —SiR₁R₂X, where X is a halogen atom. The Si—X group can bereactive for the replacement of the —X atom with another desiredfunctional group using, for example, conventional chemical reactions.Thus, a desired compatible functional group can be placed at theterminal positions. The compatible functional groups can be locatedalong the polymer side chains in addition or as an alternative toplacing the compatible groups at the terminal positions.

In some embodiments, copolymers of polysiloxane are of interest hereinwith the formula T₁-(SiR₁R₂)_(n) (SiR₃R₄)_(m)-T₂ in which R₁, R₂, R₃, R₄or combinations thereof have compatible functional groups. Thesecopolymers can be random copolymers so that the monomer units arerandomly distributed within the polymers, although suitable blockcopolymers can be used similarly. The ratio n/m indicates the ratio ofthe average distribution of the first repeat unit with R₁ and R₂relative to the second repeat unit with R₃ and R₄. Similarly, n and mare distributions in which the average n, <n>, and an average m, <m>,being at least 2. In some embodiments in which the polysiloxane polymershave polar functional side groups, no more than about 50 number percentof side groups have polar groups, in further embodiments no more thanabout 35 number percent and in some embodiments from about 5 to about 25number percent of the side groups have polar functional groups. A personof ordinary skill in the art will recognize that additional ranges ofpolar functional group quantities within the explicit ranges above arecontemplated and are within the present disclosure.

In some embodiments, the composite comprises a blend of two or morepolysiloxane polymers. In general, at least one polymer of the polymerblend comprises compatible functional groups. The average molecularweights of the different polymers of the blend can be selected toprovide desired properties for the resulting composite. In someembodiments, one of the polymers of the blend is an oligomer, and theoligomer can comprise a compatible functional group at a terminalposition and/or at a side chain position. For embodiments in which thereis a blend of a polysiloxane polymer and polysiloxane oligomers, the“polymer” in these embodiments refers to polysiloxane molecules with anaverage of at least 11 repeat units in the molecule such that oligomerscompositions and polymer compositions are distinguished.

Compatible functional groups for the polymer are evaluated relative tothe surface properties of the inorganic particles. In particular, if theparticle surface with or without modification is polar, the compatiblefunctional group of the polymer generally is polar. In otherembodiments, the inorganic particles with or without surfacemodification are hydrophobic, so the compatible functional groups shouldbe hydrophobic. In further embodiments, the inorganic particles havesurface modification with an aromatic functional group so that acompatible functional group would also have an aromatic functionalgroup. In particular, aromatic functional groups align with stableinteractions.

In the examples below, it is found that metal oxide particles formedusing a flow based technique have polar surface properties in anunmodified form which are compatible with polar polysiloxane polymers.Similarly, metal oxide particles modified with a surface modifying agentcomprising aromatic functional groups are compatible with polysiloxanepolymers comprising aromatic functional groups. These relationships alsoprovide guidance for the formation of suitable polysiloxane polymerblends. For example, unmodified nanoparticles with a polar surface canbe combined with a polysiloxane polymer blend in which at least onepolymer of the blend has polar functional groups. The polysiloxanepolymers tend to be compatible with each other even if they comprisedifferent functional groups since the polymer backbones also contributesignificantly to the polymer properties in a way that generally supportsthe compatibility of the polymers with each other. Through the use of apolymer blend the overall properties of the composite can be adjusted toachieve desired properties while still being able to stably incorporatethe particles within a well dispersed composite composition.

In general, the surface chemistry of the inorganic particles can beevaluated by forming a liquid dispersion with the inorganic particlesand evaluating whether or not the inorganic particles can be welldispersed within the liquid. A good dispersion is found if the inorganicparticles with or without surface modification can be dispersed with atleast 5 weight percent of the inorganic particles after applying goodmixing conditions with the particles remaining suspended without furthermixing for at least about 2 hours. Through the evaluation of thedispersability of the inorganic particles in different solvents, aperson of ordinary skill in the art can correspondingly identifysuitable functional groups of a polymer that would be compatible withthe inorganic particles. Similarly, polymers with the functional groupsoften are soluble in corresponding solvents, although polymers can bemore complicated due to the potential presence of portions of thepolymer with different chemical properties. The polymer properties canbe evaluated based on the chemical formula if it is known since theproperties of particular functional groups can be evaluated by a personof ordinary skill in the art. If the chemical formula of a polymer isnot know or only partially known, the properties of the polymer can alsobe evaluated based on its interaction with a solvent similar to theinorganic particles. While polymers can be more complicated, thesolubility of a polymer in particular solvents can give significantamounts of information regarding at least some significant functionalgroups of the polymer.

As a particular example, metal oxide and metal nitride particles canhave polar surface chemistries if appropriately synthesized and/orprocessed following synthesis. Polar nanoparticles can then bestabilized within a polymer with polar functional groups. Polarfunctional groups have a permanent electric dipole moment due to anon-uniform distribution of positive and negative charges. In general,polar functional groups of particular interest can have a dipole momentof at least about 0.5 Debye. The dipole moment of a functional group canbe evaluated by replacing the polymer other than the particularfunctional group with a CH₃ group and evaluating the dipole moment ofthe resulting molecule. Suitable polar functional groups include, forexample, —ROH, —RNH₂, —RSH, —RCN, —RNO, —RCOOH, and —RX, where X is F,Cl, Br or I and R is an organic group, such as an alkyl, alkenyl,alkynyl, aryl, combinations thereof, substituted forms thereof, branchedformed thereof or the like. Similarly, aromatic groups associated withthe particles can improve compatibility with polymers also comprisingaromatic groups.

As described above, it has been discovered that there are advantages inimproving the uniformity of the distribution of nanoparticles throughoutthe polymer within the composite. Achieving desired levels of uniformitycan be facilitated through the processing to form the composite. It hasbeen discovered that the introduction of appropriate functional groups,in particular compatible functional groups, can overcome thesedifficulties to achieve desired levels of uniformity of the dispersionof particles within the polysiloxane polymers. This compositeengineering can result in very desirable materials that exhibit uniqueproperties, such as a high level of transparency with a high index ofrefraction.

Generally, it is desirable to form good dispersions of the nanoparticlesfor processing into the composites. Generally, the ability to dispersethe inorganic nanoparticles is significantly dependent on the selectionof dispersing liquid. Generally, the selection of a good dispersingliquid depends on whether or not the nanoparticles are surface modified.It has been found that metal oxide particles produced by flow-basedsynthesis approaches can be well dispersed at moderate concentrations inunmodified form in alcohols, such as methanol or propylene glycol. Theas-synthesized particles can be stably dispersed in water at moderateconcentrations, although the average secondary particle size generallyis larger than in dispersions in certain alcohols.

The performance of surface modification to obtain well dispersed surfacemodified particles is described further in copending U.S. patentapplication Ser. No. 11/645,084 to Chiruvolu et al., entitled“Composites of Polymers and Metal/Metalloid Oxide Nanoparticles andMethods for Forming These Composites,” incorporated herein by reference.For example, alkoxysilanes can be used to covalently bonded with metaloxide particles. In some embodiments, if the particles are welldispersed prior to surface modification, correspondingly well dispersedsurface modified particles can be obtained with little or noagglomeration. In general, surface modified particles can be welldispersed at higher concentrations than the unmodified particles.However, nanoparticles without surface modification can be effectivelyincorporated in the composites described herein with good uniformity andhigh loadings using the approaches described herein.

While in principle, the incorporation of well dispersed particles into apolymer composite seems straightforward, it can be complex to properlydisperse the particles uniformly through the composite material. A firstcomplication is that the solvent suitable for the polymer may not besuitable for the particle dispersion. The particle dispersions generallyare very sensitive to the liquid properties. A second complication isthat the particles tend to clump in the composite even if the particlesare well dispersed prior to forming the composite. It has beendiscovered that if the polymer and inorganic particle surface propertiesare properly selected to be compatible, the polymer interacts in astable way with the particles so that the particles if well dispersed inthe polymer remain well dispersed. In particular, the particles can formstable formations with the polymer whether or not the particleschemically bond with the polymer. However, if the particles getsignificantly agglomerated, they can be difficult subsequently todisperse the particles uniformly in the polymer.

Also, the polymer molecules can have a structure in solution that candiscourage incorporation of the particles into a uniform structure. Forexample, the polymer molecules can fold to form nanoscale polymerparticles within the solution. The folding of the polymer moleculeswithin the solution can further discourage uniform blending of theparticles within the composite since only the surface of the polymerparticle may be exposed. Selection of a solvent to denature tertiarystructure of the polymer molecules can encourage formation of a uniformcomposite. The molecular weight distribution of the polymer and theconcentration of the polymer determine the viscosity of the polymersolution in a selected solvent. A lower viscosity improves the mixingwith inorganic particle dispersions. For a fixed molecular weight, thesolvent and concentration can be adjusted to provide a viscosity in adesired range. The mixing conditions should be selected to provide anappropriate amount of time and an environment during the mixing processfor macromolecules to re-configure and for particles to incorporate intothe polymer macromolecules. For composite embodiments having a polymerblend, the nanoparticles can be blended first with the polymer havingcompatible functional groups prior to or at the time of blending withthe polymer.

Furthermore, the uniformity of the composite can be significantlyimproved through the gradual addition of the nanoparticle dispersion toa solution of the polymer while mixing the polymer solution. Thedrop-wise or other gradual addition of the nanoparticle dispersionleaves a low concentration of nanoparticles within the combined solutionin which the nanoparticles are not yet associated with the polymer. Asinorganic nanoparticles are associated with the polymer, these particlesare generally not available to form agglomerates with other inorganicnanoparticles. Thus, through the gradual addition of the nanoparticledispersion with the polymer solution, the nanoparticles can be veryuniformly distributed through the composite so that on smaller distancescales the material has the appearance of a uniform material. Theappropriate slowness of the addition generally depends on the mixingconditions, and an appropriate rate of mixing can be evaluatedempirically based on the teachings herein.

In some embodiments, the dispersed nanoparticles can be blended withmonomers or oligomers in a solvent prior to polymerization. Thepolymerization can be initiated during the addition of the particledispersion or after the particles are blended with the polymerprecursors. Polymerization or crosslinking can be initiated through theaddition of a reactant, such as a radical initiator, a catalyst,radiation, combinations thereof or the like.

Solvent stability issues arise with the addition of the particles to apolymer, oligomer or polymer precursor solution since the nanoparticlesmay not form a stable dispersion with liquids that are suitable todissolve the polymer. In general, a dispersing liquid used for thenanoparticles can be selected to be miscible with the solvent for thepolymer, oligomer or the polymer precursor. Proper selection of thesolvent reduces agglomeration when the particles are added to thepolymer materials. This selection of dispersing liquid improvesdispersion of the particle through the solvent even for embodiments inwhich the particles are added gradually to the polymer solution.

Following blending the desired components of the composite, thecomposite can be further processed using polymer processing techniquessuch as casting, molding, extruding, calendering, and the like.Similarly, coating techniques can be used to apply the composite tovarious structures. Suitable coating techniques include, for example,extrusion, spin coating, spray coating, dip coating and the like. Thesolvent can be removed if desired through evaporation or otherappropriate techniques.

The composite materials with improved uniformity described hereinprovide for significantly improved optical properties, such asrefractive index matching at either higher or lower index withoutsacrificing the optical transmission property. In particular, since thematerials can be significantly more uniform on a distance scale on theorder of the wavelength of visible and/or infrared light, the compositecan be significantly more transparent to light with an appropriatewavelength. While not wanted to be limited by theory, reasonablearguments suggest that increased scattering of less uniform compositesresults from clustering of the particles or similarly by microscopicindex of refraction variations that are roughly on the order ofmagnitude of distances of the wavelength of light. Nevertheless, theimproved uniformity of the composite achieved herein provides for atransmittance of visible light greater than 90 percent relative to thetransmittance through a corresponding structure formed with the polymerwithout the particles, in which the structure is a 100 micron thickfilm.

Other optical properties of the composite can be similarly improved. Forexample, the formation of composites with phosphor particles can resultin improved luminescence of the composites due to the improveduniformity of the materials. A desirable mixture of particles cancomprise, for example, phosphor particles along with particles thatincrease the index of refraction. Other functional optical materialssimilarly have improved performance due to the improved uniformity ofthe composite.

EXAMPLES Example 1 Composite Film of TiO₂ in AS4000 Resin

The example demonstrates the ability to form highly transparent filmswith a polar polysiloxane resin loaded with TiO₂ particles withoutsurface modification of the inorganic particles.

Rutile TiO₂ Methanol Dispersion

Rutile TiO₂ was produced by laser pyrolysis essentially as described inExample 1 of U.S. patent application Ser. No. 11/645,084 filed Dec. 22,2006 to Chiruvolu et al., entitled “Composites of Polymers andMetal/Metalloid Oxide Nanoparticles and Methods for Forming TheseComposites,” incorporated herein by reference. The TiO₂ particles had aprimary particle size as determined using a transmission electronmicrograph of 7-10 nm, and a BET surface area >160 m²/g.

TiO₂ particles were dispersed in methanol to form a 10% wt dispersionand milled with a Netzsch MiniCer with 0.1 mm YTZ® beads as millingmedia. The size of the particles in the dispersion after milling wasmeasured with a Malvern ZetaSizer™. As shown in FIG. 1, the size of theparticles in the dispersion is small, with a Z-average particle size of16.9 nm.

AS4000 Resin

AS4000 resin is a clear, abrasion-resistant silicone hard coat resincommercially available from Momentive Performance Materials (CT, USA).Unlike some other polysiloxanes, AS4000 resin is a polar polymer that iscorrespondingly soluble in polar solvents. The resin obtained from themanufacturer is dispersed in a mixture of solvents including methanol,n-butanol and isopropanol.

Composite Film Preparation and Properties

Rutile TiO₂ methanol dispersion (10% wt) prepared above was mixed withAS4000 resin to form composites with 50% wt or 60% wt TiO₂. Therespective composites were spin coated using a Laurell WS-400B-NPP-Liteinstrument to form films on glass or a silicon wafer. The transmittancespectra of the films were taken on a SINCO UV-vis spectrometer, using aglass slide as reference. The refractive index value and the thicknessof the films were measured on a SCI FilmTek 3000 instrument, with allthe RI value measured at 632 nm.

FIG. 2 shows the transmittance spectra of three films in visible range,all of which has higher than 90% transmittance. Film 1 is neat siliconecoating of AS4000 resin without TiO₂ with a RI at 632 nm of 1.44. Film 2is 50% wt TiO₂ composite, with a RI at 632 nm of 1.60 and a thickness ofabout 333 nm Film 3 is 60% wt TiO₂ composite, with a RI at 632 nm of1.66 and a thickness of about 645.4 nm.

Example 2 Composite Film of Surface Modified TiO₂ inPolydimethylsiloxane (PDMS)

The example is a comparative example with the highly transparent filmsof Example 1.

TiO₂ Surface Modification with Hexamethyldisilazane &Octamethylcyclotetrasiloxane

Rutile TiO₂ was produced by laser pyrolysis essentially as described inthe above Example 1. A blend of hexamethyldisilazane (HMDZ, SigmaAldrich) & octamethylcyclotetrasiloxane (D4, Gelest Inc) was used asparticle surface modification reagent. Rutile TiO₂ was modified withHMDZ&D4 in methyl ethyl ketone (MEK) through bead milling process toproduce a 3% wt dispersion following the procedure described below. TiO₂was first treated with HMDZ in MEK for one hour and then introduced intoa bead mill. During the milling process, HMDZ and D4 were added into themilling mixture in three steps. The resultant secondary particles sizeswere evaluated with dynamic light scattering (DLS) using a MalvernZetaSizer™ instrument. The distribution of the size of the particlesrevealed from DLS measurement is shown in FIG. 3 where majority of thesecondary particles had a diameter of 110.2 nm. A smaller group ofparticles had a peak secondary particle size of about 12.45 nm. TiO₂particles therefore formed a good dispersion in MEK at 3% wt aftermodification with HMDZ&D4.

Composite Film Preparation

A 0.7 g quantity of the surface modified TiO₂ dispersion in MEK preparedas described above was mixed with Gelest OE™ 41 2-part flexible opticalencapsulant (OE™ 41, Gelest, Inc., main component is PDMS, part A 0.65 gand part B 0.65 g) and sonicated for 0.5 hr. The MEK was then removed byrotary-evaporation to form a liquid. The liquid was then casted ascomposite film on glass with a blade coater to get a 40 μm thickness.The film was then cured at 150° C. for 2 hours. The content of TiO₂ inthe composite films was about 35% wt.

FIG. 4 shows the transmittance spectra of the composite film.Specifically, transmittance spectra of TiO₂-PDMS silicone film (40 μmthickness) with loading of 35% wt TiO₂ was taken. The transmittance ofthe film in visible range is low due to poor compatibility of TiO₂ withPDMS even following surface modification of the titania particles. Sincethe functional groups of the surface modifying agents and the sidegroups of the polymer are hydrophobic organic groups, these resultsimply that the polymer backbone significantly influences the propertiesof the polymer. Evidently, more dominant functional groups, such aspolar groups or aromatic groups, on the polymer can provide forstabilization of interactions with appropriate inorganic particles.

The embodiments above are intended to be illustrative and not limiting.Additional embodiments are within the claims. In addition, although thepresent invention has been described with reference to particularembodiments, those skilled in the art will recognize that changes can bemade in form and detail without departing from the spirit and scope ofthe invention. Any incorporation by reference of documents above islimited such that no subject matter is incorporated that is contrary tothe explicit disclosure herein.

1. A method for forming a composite, the method comprising: combiningwell dispersed inorganic particles in a dispersant liquid comprising apolar solvent with a first polysiloxane in a suitable solvent differentfrom the dispersant liquid and miscible with the dispersant liquid forsolubilizing the first polysiloxane to form a blend, the inorganicparticles having an average particle diameter of no more than about 250nm and being substantially free of organic surface modification distinctfrom the polysiloxane, wherein the inorganic particles comprise a metaloxide, a metal nitride or a combination thereof, wherein the firstpolysiloxane comprises functional side groups, and wherein about 5number percent to about 50 number percent of the functional side groupscomprise aromatic functional groups or polar functional groups selectedfrom the group consisting of —ROH, —RNH₂, RSH, —RCN, —RNO, RCOOH, and—RX, where X is F, Cl, Br, or I and R is an organic group.
 2. The methodof claim 1 wherein the first polysiloxane comprises compatiblefunctional groups that have stabilizing interactions with the inorganicparticles.
 3. The method of claim 1 wherein the inorganic particlescomprise a metal oxide.
 4. The method of claim 1 wherein the inorganicparticles were synthesized in a flow based process.
 5. The method ofclaim 1 wherein the inorganic particles are dispersed in an alcohol. 6.The method of claim 1 wherein the polysiloxane comprises a polarfunctional group.
 7. The method of claim 1 further comprising combininga second polysiloxane with the blend of the inorganic particles and thefirst polysiloxane.
 8. The method of claim 1 further comprising formingthe blend into a desired structure and drying the blend to remove theliquid.
 9. The method of claim 1 wherein the inorganic particlescomprise a mixture of inorganic particles.
 10. A method for theformation of a composite, the method comprising: combining welldispersed inorganic particles in a dispersant liquid comprising a polarsolvent with a first polysiloxane in a suitable solvent miscible withthe dispersant liquid for solubilizing the first polysiloxane to form ablend, the inorganic particles having an average particle diameter of nomore than about 250 nm and being free of organic surface modificationdistinct from the first polysiloxane, and wherein the inorganicparticles comprise a metal oxide, a metal nitride or a combinationthereof; and combining a second polysiloxane with the blend of theinorganic particles and the first polysiloxane wherein the secondpolysiloxane does not have compatible functional groups, wherein thefirst polysiloxane comprises functional side groups, and wherein about 5number percent to about 50 number percent of the functional side groupscomprise aromatic functional groups or polar functional groups selectedfrom the group consisting of —ROH, —RNH₂, RSH, —RCN, —RNO, RCOOH, and—RX, where X is F, Cl, Br, or I and R is an organic group.
 11. Themethod of claim 10 wherein the resulting composite composition has aloading of at least about 20 weight percent inorganic particles, whereinthe composite has a transparency of at least about 90 percent for aselected visible light wavelength when formed into a film having athickness of 2.5 microns.
 12. The method of claim 10 wherein theinorganic particles comprise a metal oxide.
 13. The method of claim 10wherein the first polysiloxane polymer or second polysiloxane comprisesa polar functional group.
 14. The method of claim 10 wherein the secondpolysiloxane is an oligomer.
 15. The method of claim 10 furthercomprising forming the blend into a desired structure and drying theblend to remove the liquid.
 16. The method of claim 15 wherein the blendcomprises at least 15 weight percent of the first polysiloxane polymerafter drying.